1. Field of the Invention
The present invention relates to a fuel vapor feed controlling apparatus for a lean burn type internal combustion engine for feeding fuel vapor (vapor) generated in, for example, a fuel reservoir to an intake system in response to an operational condition of the lean burn type internal combustion engine.
2. Description of the Related Art
Conventionally, a generally used engine, the fuel from a fuel injection valve is injected to an intake port so that uniform or homogeneous mixture of the fuel and air is fed to a combustion chamber in advance. An intake passage is opened/closed by a throttle valve which works in cooperation with an accelerator operation.
An amount of intake air (finally, an amount of gas uniformly mixed fuel and air) to be fed in the combustion chamber of the engine is adjusted by the open/close operation of the throttle valve to thereby control the engine output.
However, in the technology depending upon the above-described so-called homogeneous combustion, a large intake vacuum is generated in the throttling action of the throttle valve so that a pumping loss is remarkable to reduce the efficiency. In contrast, a so-called "stratified combustion" technology is well known in which the throttle degree of the throttle valve is made small and the fuel is fed directly to the combustion chamber whereby a combustible mixture is present in the vicinity of a spark plug to increase an air/fuel ratio of the portion in question to enhance ignitability. In this technology, when the engine is operated in a low load, the injected fuel is locally fed to an around the spark plug, and at the same time, the throttle valve is fully opened to execute the stratified combustion. As a result, the pumping loss is reduced and the fuel consumption rate is enhanced.
The internal combustion engine for the above-described "stratified combustion" takes combustion conditions such as a stratified combustion, a weak stratified combustion, a homogeneous lean combustion and a homogeneous combustion in this order when the load is changed from a low level to a high level, for example.
As described above, the stratified combustion means a combustion in which a mixture gas layer having a high air/fuel ratio is present in the vicinity of a spark plug to form a layer with respect to the other gas in the other portion.
The weak stratified combustion means the case where the stratified degree is small in comparison with the stratified combustion.
The homogeneous lean combustion means the case where the fuel and air are homogeneous but the ratio of the fuel is low.
The homogeneous combustion means the case where the fuel and air are homogeneous and the ratio of the fuel is high.
Also, in the case where such a "stratified combustion" takes place or the lean burn takes place, in some cases, swirls would be formed in the mixture of the injected fuel. Namely, a swirl control valve (SCV) is provided in the intake port and the opening degree of the SCV is adjusted to thereby control the strength of the swirls. As a result, it is possible to enhance the combustion property with a small amount of fed fuel.
By the way, a fuel vapor feed controlling apparatus for a lean burn type internal combustion engine for temporarily storing the fuel vapor (vapor) from the fuel reservoir or the like in a canister and feeding the stored vapor into the intake system in response to the operational condition of the internal combustion engine is well known (Japanese Patent Application Laid-Open No. Hei 4-194354).
In this system, a purge control valve is interposed in a purge passage for connecting the canister for fuel vapor adsorption and the intake passage for fuel evaporation. Then, the purge control valve is controlled so that a suitable amount of fuel purge (which is an amount of the vapor introduced into the intake passage, and which will hereinafter be referred to as a purge amount) may be obtained in response to the operational condition of the engine (for example, in the case where the engine load is large, the vapor is fed).
However, in the lean combustion region, since the device for detecting the air/fuel ratio is not provided, in many cases, there is no index or factor for controlling the fuel purge amount.
More specifically, in the conventional internal combustion engine, an air/fuel ratio sensor such as an oxygen sensor or the like is usually interposed in the exhaust passage, and the actual air/fuel ratio is detected on the basis of the output signal therefrom. A fuel injection amount or the like is suitably controlled in a feed-back manner so that the air/fuel ratio of the mixture separately calculated may be the target air/fuel ratio. However, in the above-described oxygen sensor, the detection is performed around the target air/fuel ratio (A/F) of, for example, 14.5. In the case where the air/fuel ratio exceeds this, it is impossible to detect the purge amount.
For this reason, when the fuel vapor feed amount is controlled in such a lean combustion region, in the case where the air/fuel ratio is not detected or in the case where the precision of the detected air/fuel ratio is worse, the precision of the calculation of the purge amount becomes worse. Then, if the fuel vapor feed controlling apparatus is controlled in accordance with the purge amount determined by a vacuum pressure, there is a fear that a misfire or a surge would be generated when the vapor is rich.
Also, the case where the load of the engine is shifted from a high level to a low level means the same as the case the combustion condition is shifted from the homogeneous combustion or homogeneous lean combustion to the stratified combustion or the weak stratified combustion or the like. In such a case, the purge-prohibition is set. The combustion condition is unstable by the purge gas fed to the combustion chamber with a time lag due to the purge transfer delay through the intake pipe when the combustion conditions are changed. As a result, there is a fear that a rich misfire and a surge would be generated.